3d spiral with animation nodes
That can be something like this:

The idea is to use Fermat's spiral calculation twice: at the angle and at the angle plus 2xPi.
That gives two points from which we can calculate the distance and in consequence the radius of the curve vertex.

As we calculate twice, the formula in inside a group (subprogram).
The radius is empirically scaled and clamped.
Additionally, the curve U resolution is set to 1 to avoid unnecessary geometry and artifacts.
In order to have the reversed part:

Take the previous result, reverse it and combine the flipped reserved and the initial result.